Web messaging sdk startchat not passing crm id to backend

stuck on injecting the crm customer_id into the web messaging sdk startchat method. the payload seems to drop the custom_data object before hitting the api.

this.sdk.startChat({
 customData: { crm_id: '12345' }
});

no error thrown but the backend webhook receives an empty custom_data. is the sdk stripping this field or do i need to use a different init method?